Nelson Bio Chapter 9

 The brain operates on the amount of electrical power that would light a 10 watt bulb  Your brain weighs 1/50 of your adult weight and uses 25% of the oxygen you take in  A nerve cell can send 1000 impulses a second  Neurons are the largest cells in the body – some are up to 3 feet long.  There are more nerve cells in the human brain than there are stars in the Milky Way

 Nervous system contains 2 types of cells:  Glial cells – non conducting cells which provide structural support  Neurons – the nerve cell itself  1) sensory(afferent) – sense & relay info(stimulus) to the CNS  2) interneurons – link neurons within the body  3) motor neurons (efferent) – relay info to the effectors

 Glands or muscles that cause things to happen.  So why are they called effectors?

 1) cell body – contains the nucleus and cell organelles; normal cell processes happen here  2) dendrites – short, highly branched fibers specialized for receiving impulses (stimuli)  3) axon – a long thin fiber(extension of the cytoplasm) which carries impulses away from the cell body to the effectors

 More than 100 axons could be placed inside the shaft of a single human hair  Nerves are made of many neurons held together

 The synapse is the junction between the terminal branches of a neuron and another cell.  Impulses are transmitted across this gap  An axon may synapse with 1000 other neurons

 Nerve gas, botulism, some insecticides  Drugs  Stimulants –produce a feeling of well being, alertness and excitement –amphetimines bind to certain recept0rs so mimic norepinephrine, caffeine  Depressants – slow down body activites – barbituates block the formation of norepinephrine

 A nerve impulse is both an electrical and a chemical event  A nerve at rest has a relative negative charge inside and a positive charge outside  K ions are concentrated inside the neuron and Na ions are concentrated outside the neuron

 This neg. charge is maintained by sodium- potassium pumps which control the movement of Na and K ions through the cell membrane. (ATP supplies the energy to fuel the pumps).  The membrane is now said to be polarized  An external stimulus changes the ability of the membrane to keep out the Na ions & they rush in

 This rapid inflow of Na causes a charge reversal – depolarization  This change in charge stimulates adjacent points in the nerve cell & a wave of impulse passes along the nerve cell  Animation Animation  An impulse ends when K ions rush to the outside and repolarize the membrane

 A nerve impulse can travel at the rate of 100m/sec  The recovery period is called the refractory period.  Saltatory conduction is when the impulse jumps from one node of Ranvier to the next – so this is faster in myelinated neurons

 The impulse causes chemicals called neurotransmitters to be release from the vesicles in the synaptic knobs  The chemicals travel across the synaptic gap and cause an impulse to be started along the postsynaptic membrane  Animation Animation

 A stimulus must be above a certain level to produce a response  The all or none response- neurons either fire or not at all ( like a trigger of a gun)  What causes a greater intensity of response?  it is not stronger stimuli, but more impulses.

 On pg. 426 – do questions 4,5,7,8,9 and 13.